A satellite launched with fanfare by a defiant North Korea appears to be dead as no signal can be detected, according to a US-based astrophysicist who monitors spaceflights.

The United States and its Asian allies have acknowledged that North Korea succeeded in putting an object into orbit last week.

It was blasted into space by a rocket, which was condemned by the West amid fears it was a veiled test for a long-range ballistic missile.

The communist state says the satellite is observing Earth and airing patriotic songs.

But Jonathan McDowell of the Harvard-Smithsonian Centre for Astrophysics says the satellite is clearly in orbit, but no songs can be heard.

"To the best of our knowledge, the satellite isn't operating," he said.

"It's definitely up there and it's whizzing around, but it's just not feeling very well."

Mr McDowell says it is unclear whether the satellite - called the Kwangmyongsong-3 - worked initially.

He added it remains possible it is transmitting at a level too faint for detection.

But in another sign of trouble, Mr McDowell says the satellite is fluctuating in brightness.

That means the Sun is shining at different angles and the satellite is not pointing down at the Earth as it should.

Even if not functioning, the satellite remains in orbit.

One website tracked the satellite as orbiting at least 505 kilometres above Earth, in line with North Korean statements.

"These things are hard to calculate, but roughly speaking, an object of that density at that height is going to stay up for a few years," Mr McDowell said.

The state-run Korean Central News Agency last week quoted a scientist saying the technology was "flawless" and the satellite was broadcasting "Song of General Kim Il-Sung" and "Song of General Kim Jong-Il," references to the state's first two leaders.